1. Field of the Invention
This invention relates to an anti-skid control apparatus for a vehicle braking system which can prevent the locking of the wheels, and more particularly to an anti-skid control apparatus which controls brake pressures to the wheel cylinders in accordance with the skid conditions of the wheels.
This applicant proposed previously the anti-skid control apparatus in the Japanese Patent Application No. 134665 (U.S. Ser. No. 57,802 filed June 3, 1987, now U.S. Pat. No. 4,740,040) in order to provide an anti-skid control apparatus for a vehicle braking system which can be small-sized and light, and can avoid the fear of locking of rear wheels in any cases. In the above apparatus, two fluid pressure control valve devices are used (so-called "2-channel"), and wheel speed sensors are provided for the respective wheels, which are four. And it is decided from the judging results of the skid conditions of the rear wheels which side of the road, on which the wheels are running, is frictionally lower between the sides of the road.
It is considered that the number of the wheel speed sensors is reduced to three for reducing further the cost of the apparatus. The number of the labors for assembling and mounting the wheel speed sensors can be reduced with the reduction of the number of the wheel speed sensors to be provided, too. However, when the wheel speed sensors are three, there occurs the following inconvenience.
When wheel speed sensors are provided for the front wheels respectively, and one common wheel speed sensor is provided on a differential mechanism (rear differential) for the rear wheels which is arranged in a rear axle for connecting the rear wheels, it is difficult to detect the rotational speed difference between the rear wheels and it is impossible to decide the frictionally low side of the road by the prior art method. Or when wheel speed sensors are provided for the rear wheels respectively, and one common wheel speed sensor is provided on a differential mechanism (front differential) for the front wheels which is arranged in a front axle for connecting the front wheels, it is difficult to detect the rotational speed difference between the front wheels, although the rotational speed difference can be detected between the rear wheels. However, the low side cannot be decided from both of the rear wheels and of the front wheels by the prior art method.